The present invention relates to a nickel hydroxide which is stable to oxidation and coated with cobalt hydroxide, and to a process for the preparation thereof, in particular for use as a positive active mass in rechargeable alkaline batteries.
Although nickel hydroxide is outstandingly suitable for storage of electrical energy because of its storage capacity of 1 to (with a corresponding defect structure) a theoretical maximum of 1.67 electrons per Ni, it has a number of undesirable properties, such as low electrical conductivity, low cycle stability, low charging capacity at high temperatures and the tendency to swell because of the spontaneous formation of different crystal phases with different lattice spacings.
In a very early stage of the development of nickel hydroxide batteries it was already recognized that the properties of the batteries can be improved by using nickel hydroxide coated with cobalt(II) hydroxide (see U.S. Pat. No. 3,066,178). Nevertheless, the industrial use of nickel hydroxide coated with cobalt hydroxide has not since become accepted because of the sensitivity of cobalt(II) hydroxide to oxidation. Rather, the route taken in the production of the battery is the use of non-coated nickel hydroxide with the addition of Co metal powder or cobalt compounds, such as Co(II)O or Co(OH)2, where a cobalt hydroxide coating crosslinked between the nickel hydroxide particles forms on the nickel hydroxide via intermediate cobalt(II) hydroxo complexes during more prolonged standing (typically 1 to 3 days), the cobalt compound dissolving in the electrolyte, this coating then being converted into the actually conductive but electrochemically inactive cobalt oxyhydroxide network during the first electrical charging of the battery. A comprehensive description of the mechanisms which are important here is to be found in the paper by Oshitani at the 3rd Symposium for Sectional New-Battery Study Group in Battery Technology Committee of the Electrochemical Society, Dec. 11, 1986, entitled xe2x80x9cDevelopment of high-capacity nickel-cadmium battery using sintered metal fiber as substratexe2x80x9d. The models described there in respect of cobalt compounds also apply to other substrates and cathode materials.
When nickel hydroxides which are coated according to the prior art with a cobalt(II) hydroxide layer which is not stable to oxidation by atmospheric oxygen are used, in the course from production via storage to the actual use in the battery a passivating cobalt(II)-containing layer forms on the surface of the coated nickel hydroxide particles, this not only impeding the solubility of the cobalt species to give the cobalt hydroxo complexes to be formed intermediately (reduction in the contact surface due to inadequate fusion), but additionally having a poor electrical conductivity. This then necessarily means that large parts of the active nickel mass are not accessible electrically and as more or less dead material can no longer contribute towards the capacity of the battery.
It has also already been proposed (Japanese Patent 25 89 123) to generate the conductive electrochemically inactive cobalt oxyhydroxide layer by precipitating a cobalt hydroxide layer on to the nickel hydroxide particles and subsequently oxidizing the layer in alkaline solution by means of oxygen at a higher temperature. Such nickel hydroxide coated with cobalt oxyhydroxide is indeed stable to oxidation; however, a disadvantage is the fact that although the individual particles in its outer shell have a good conductive layer, the formation of a three-dimensional conductive network between the individual particles (increasing the contact surface by xe2x80x9cfusionxe2x80x9d) can be achieved only by further addition of cobalt compounds which are adequately soluble in the alkaline electrolyte. If this addition is omitted, only loose contact points exist between the individual particles and the total resistance of the electrode is increased because of the transition resistance which occurs between the individual particles.
On the other hand, a cobalt(II) hydroxide coating remains soluble in the alkaline electrolyte to the extent that crosslinking takes place by dynamic dissolving and addition processes, so that conductivity bridges are generated between the particles after conversion to cobalt oxyhydroxide during the initialization charging cycle. The coated individual particles are xe2x80x9cfusedxe2x80x9d in an electrically conductive manner at the contact points of the Co(OH)2 or CoOOH coating. A condition of this, however, is the provision of a nickel hydroxide with a cobalt(II) hydroxide coating which is stable to oxidation.
Accordingly, it has also already been proposed to treat cobalt(II) hydroxide (as a conductive additive to nickel hydroxide) or nickel hydroxide coated with cobalt(II) hydroxide with antioxidants, such as D-glucose (EP-A 744 781) or higher carboxylic acids, their esters, aldehydes, phenols or vitamins (EP-A 771 041). A disadvantage here is that the protection against oxidation is achieved only indirectly, since the antioxidants only become active with respect to the oxidized cobalt(II) form in the sense of a reduction. A further disadvantage is that the antioxidant is consumed in the course of time, that is to say the oxidation protection is limited with respect to time. There is furthermore the risk that undesirable degradation products of the antioxidant are entrained into the battery.
It has now been found that the cobalt hydroxide layer can be formed in a form which is stable to oxidation if the coated nickel hydroxide is treated with weak inorganic oxygen acids or alkali metal salts thereof. In this procedure, the surface of the coated particles is covered with anions of the acids. Suitable anions are one or more anions from the group consisting of aluminate, borate, carbonate, chromate, manganate, molybdate, niobate, phosphate, silicate, tantalate, titanate, vanadate and tungstate, or oxalates.
Borate, phosphate, carbonate and/or silicate are preferred. Carbonate is particularly preferred.
xe2x80x9cStable to oxidizationxe2x80x9d in the context of the invention means that the degree of oxidation of the cobalt hydroxide does not change during storage in air and at ambient temperature (up to 40xc2x0 C.). Li, Na, K and/or pseudo-alkali metals, such as ammonium, are suitable as the alkali metal. The treatment is preferably carried out in an aqueous solution of the alkali metal salts, in particular of the sodium salt.
A less than monomolecular covering of the Co(OH)2 surface is sufficient to render the nickel hydroxide provided with the Co(OH)2 coating stable to oxidation.
It has been found that such a surface covering does not impair the solubility of the cobalt(II) hydroxide coating in the alkaline electrolyte of the battery, so that the effect of the formation of a three-dimensional network of high conductivity, called xe2x80x9cfusionxe2x80x9d above, can also take place unimpeded without the addition of a further conductive additive. As a result, it is possible to keep the total amount of cobalt employed to a minimum.
The invention provides a nickel hydroxide which is provided with a cobalt hydroxide layer which is stable to oxidation and the pastel green colour of which is retained during storage in the atmosphere for a period of at least 4 weeks; preferably for a period of 6 months.
The invention also provides a nickel hydroxide provided with a cobalt hydroxide layer which is stable to oxidation, the content of cobalt in the 3-valent oxidation level increasing by less than 0.5%, based on the total content of cobalt, even after storage in air (ambient temperature 10 to 35xc2x0 C.) for at least 6 months.
The oxidation level of the cobalt here is preferably determined by iodometric titration in a manner known per se.
The invention furthermore provides a nickel hydroxide which is provided with a cobalt hydroxide coating and has on its surface an at most monomolecular layer of anions of weak inorganic oxygen acids. The concentration of the anion of the weak acid is preferably 5 to 20 xcexcmol/m2 of surface coated with cobalt hydroxide. A surface covering of 10 to 18 xcexcmol/m2 is especially preferred.
The superficial anion concentration is preferably 10 to 50 mmol per mol of cobalt(II) hydroxide, based on the amount of the cobalt hydroxide coating.
The nickel hydroxide powder according to the invention is preferably free from antioxidants or organic degradation products thereof.